1. Field of the Invention
The present invention relates to an antenna device and a radar apparatus.
2. Description of the Related Art
In monopulse radar systems, an array antenna forms a beam to transmit a signal. Then, the array antenna receives an echo signal which is corresponded to the signal in order to measure a target angle.
The array antenna includes several subarray antennas as disclosed in “Antenna Engineering Handbook”, Ohmsha, pp. 339-pp. 445. In millimeter wave band, one side of each subarray antenna is connected to a feeding interface such as a waveguide or a line such as a triplate line and a microstrip line in order to feed a signal. These feeding methods are disclosed by H. Iizuka, K. Sakakibara, T. Watanabe, K. Sato, and K. Nishikawa, “Antennas for Automotive Millimeter-wave Rader Systems”, IEICE, SB-1-7, pp. 743-pp. 744, 2001, and in JP-A 2000-124727(KOKAI).
A waveguide feeding method is popular for the antenna in automotive radar systems using the millimeter wave. In the case that the width of the feeding interface which is the waveguide is larger than interval of the subarray antenna an extra space is required between adjacent subarray antennas when all feeding interfaces are formed at the same side of all subarray antennas. As a result, an aperture area of the array antenna gets large.
On the other hand, the space between the adjacent subarray antennas should be narrow in order to achieve a wide coverage angle in the automotive radar systems.
One of the waveguide feeding methods is disclosed by Y. Okajima, S. Park, J. Hirokawa, and M. Ando, “A Slotted Post-wall Waveguide Array with Inter-digital Structure for 45-deg Linear and Dual Polarization”, IEICE Technical Report, AP2003-149, RCS2003-155, pp. 21-26, 2003. In this reference, the subarray antennas in the array antennas are arranged in an inter-digital structure.
In the array antenna with the inter-digital structure, the feeding interfaces are formed at a different side of the subarray antennas alternately. Therefore, since the adjacent subarray antennas are arranged with no space, it can achieve a small aperture area of the array antenna.
However, the array antenna with the asymmetrical inter-digital structure for a scan plane causes an asymmetrical phase difference of a signal beam of each subarray antenna because of manufacturing tolerance. As a result, measurement accuracy of the target angle degrades in the monopulse radar systems using the array antenna with the inter-digital structure.